In Skubana, billing or inventory allocation zones are the collection of US states assigned to cover your warehouses. You are required to set these up if you have more than one warehouse. Otherwise, Skubana will set up a default inventory allocation zone covering the whole country for you.
When an order is placed, Skubana will look up which Inventory Allocation Shipping Zone the customer's shipping address belongs to, then deduct inventory from the warehouse assigned to that zone.
Keep in mind that your default Inventory Allocation Shipping Zones can be overriden on a per-product basis in the 'Inventory' section. In other words, you can create custom Inventory Allocation Shipping Zones for each and every product you offer, which will take precedence over your default zones.
If your business operates from a single warehouse, then you don't need to worry about Inventory Allocation Shipping Zones, as Skubana will create a default zone that covers the entire country. But, if in the future your business grows to the point where you need to add another warehouse (and we sure hope that it does!), Skubana will guide you through the process of creating default Inventory Allocation Shipping Zones.